A surface mounted circuit, for example, a hybrid circuit, may contain one or more monolithic circuits or individual circuit elements, for example, transistors, capacitors, or the like, bonded to an insulating substrate with appropriate interconnections. Hybrid circuits offer isolation between components and permit the use of precise resistors and capacitors. Moreover, hybrid circuits offer the advantage of relatively low cost for applications where only a relatively small number of circuits are built. Additionally, for certain applications, hybrid circuits offer greater thermal stability integrated circuits.
In surface mounted circuits, for example, hybrid circuits, passive elements are fabricated and interconnected by either thick film processes or thin film processes. While these terms are relative, they are fairly well understood operationally.
In thick film circuits, resistors and interconnect patterns are printed on the insulating substrate by silk screen or similar processes. Pastes, for example, pastes of powder such as silver powders in organic binders, such as epoxy resins, are printed on the substrate, for example, through silk screens, and thereafter cured in an oven. This process allows resistors to be made having a resistance below the rate of value and then having their resistance increased, for example, by abrasion or ablation. Such corrections of resistance can be made quickly with automated procedures, wherein the resistance values are continually monitored during manufacturing. Similarly, ceramic chip depositors can be bonded into place in the interconnect pattern along with monolythic circuits and individual transistors.
In surface mounted circuits, including hybrid circuits, thin film technology allows for greater precision and miniaturization. Thin film interconnect patterns and circuit elements, as resistors, have heretofore been vacuum deposited, as by sputtering, on a glass or glazed ceramic substrate. The resistive films have heretofore been made by tantalum or another resistive metal, and the conductors and interconnects by aluminum or gold. In general, resistive materials are deposited by sputtering and pattern definition for the resistors and conductive paths, i.e., interconnects, is achieved by depositing the films through metal shields which contain appropriate openings or apertures.
While hybrid circuits offer higher reliability and lower cost in integrated circuits, full utilization of hybrid circuit technology is limited by the relatively thick geometry, the line wide definition being on the order of about 125 microns.